A microwave multistage amplifier is known in which two or more amplifiers having the same characteristics are connected with a transmission line in a cascade manner and which includes a unit configured to adjust a line length of the transmission line (see Japanese Laid-open Patent Publication No. 63-46007).
A notch filter for forming an attenuation region between pass regions formed on a low frequency side and a high frequency side is known (see Japanese Laid-open Patent Publication No. 2012-175438). Plural phase-inversion inductors are connected in series as a series arm between an input port and an output port. First and second electrode portions are formed on one surface and the opposite surface of an insulating plate. A conduction path is formed in the insulating plate so as to connect the first electrode portion and the second electrode portion to each other. Plural element portions are parallel arms which cause series resonance at a frequency corresponding to the attenuation region, one end of which is connected between neighboring series arms and the other end of which is connected to the first electrode portion. A capacitance component is installed between the element portion and the second electrode portion and the capacitance value thereof is set so as to cause series resonance with an inductance component of the conduction path at a frequency corresponding to the attenuation region.
[Patent Document 1] Japanese Laid-open Patent Publication No. 63-46007
[Patent Document 2] Japanese Laid-open Patent Publication No. 2012-175438
A gate-grounded amplifier has a problem in that the gate-grounded amplifier oscillates at a predetermined frequency by positive feedback of a return current. A source-grounded amplifier has also the same problem. It is possible to prevent oscillation of an amplifier using a notch filter. However, since the notch filter attenuates a signal of a predetermined frequency band, there is a problem in that power loss is caused and a gain in the predetermined frequency band decreases.